Chapter 12 - Power Protection Functions
Compressor load on single shaft machines leads to a high motoring power compared to split-shaft machines. Rapid
disconnection is required to limit power loss or damage.
Power is low when blades are above tail-race water level. Hydraulic flow detection devices are often the main means of
detecting loss of drive. Automatic disconnection is recommended for unattended operation.
Damage may occur rapidly with non-condensing sets or when vacuum is lost with condensing sets. Reverse power
protection may be used as a secondary method of detection and might only be used to raise an alarm.
In some applications, the level of reverse power in the case of prime mover failure may fluctuate. This may be the
case for a failed diesel engine. To prevent cyclic initiation and reset of the main trip timer, an adjustable reset time
delay is provided. You will need to set this time delay longer than the period for which the reverse power could fall
below the power setting. This setting needs to be taken into account when setting the main trip time delay.
A delay in excess of half the period of any system power swings could result in operation of the reverse power protection
REVERSE OVERPOWER SETTING GUIDELINES
Each stage of power protection can be selected to operate as a reverse power stage by selecting the Power>(n)
Direction cell to Reverse.
The relevant power threshold settings should be set to less than 50% of the motoring power.
The operating mode should be set to Reverse.
The reverse power protection function should be time-delayed to prevent false trips or alarms being given during
power system disturbances or following synchronisation.
A time delay setting, of approximately 5 s would be typically applied.
The delay on the reset timer, Power>1 tRESET or Power>2 tRESET, would normally be set to zero.
When settings of greater than zero are used for the reset time delay, the pick-up time delay setting may need to be
increased to ensure that false tripping does not result in the event of a stable power swinging event.
Reverse overpower protection can also be used for loss of mains applications. If the distributed generator is
connected to the grid but not allowed to export power to the grid, it is possible to use reverse power detection to
switch off the generator. In this case, the threshold setting should be set to a sensitive value, typically less than 2%
of the rated power. It should also be time-delayed to prevent false trips or alarms being given during power system
disturbances, or following synchronisation. A typical time delay is 5 seconds.
10% - 15% (Split-shaft)
0.2 - >2% (Blades out of water)
>2.0% (Blades in water)
0.5% - 3% (Condensing sets)
3% - 6% (Non-condensing sets)
Possible damage (percentage rating)
With some gear-driven sets, damage may arise due
to reverse torque on gear teeth.
Blade and runner damage may occur with a long
period of motoring
Thermal stress damage may be inflicted on low-
pressure turbine blades when steam flow is not
available to dissipate losses due to air resistance.